Turn and phase counts of individual motor unit potentials: correlation and reliability.

Different turn algorithms are used for quantitative motor unit potential (MUP) analysis. To compare their retest reliability, 420 myopathic and neuropathic MUPs were recorded twice and the turn count of the first registration was correlated with that of the second. Reliability was best for the algorithm according to Willison as compared to the conventionally used algorithms based on amplitude criteria for 2 or 3 successive relative extrema. As demonstrated by discriminant analysis, an amplitude limit of 25 microV yielded more useful turn counts than a limit of 50 microV if myopathic MUPs had to be discriminated from normal MUPs. For this discrimination the turn count was superior to the phase count which did not further improve the discriminant model. This was different for the discrimination between normal and neuropathic MUPs. In this case, both parameters measured partly independent features of the MUP and had to be considered together.